Method and apparatus for generating an improved image of natural tissue in reconstructing body images from 3D-measurements

ABSTRACT

A method is disclosed for generating images of a human or animal body on the basis of 3D-reconstructions from 3D-XRAY or 3D-Computer Tomography measurements, which bodies comprise both natural tissue and one or more high-density objects. The method comprises the steps of executing the measurements, distinguishing the one or more high-density objects and executing a separating procedure thereon for generating an improved image of regions of the natural tissue. In particular, the method comprises the following steps: applying a ramp filter on the various projection measurements to single out the one or more high-density objects; segmenting the singled-out one or more high-density objects into a separate 3D reconstruction; suppressing the reconstructed one or more high-density objects from the original projection measurements; and segmenting said projection measurements without the suppressed one or more high-density objects.

BACKGROUND OF THE INVENTION

The invention relates to an apparatus for generating images of human oranimal bodies on the basis of 3D-reconstructions from 3D-XRAY or3D-Computer Tomography measurements, which bodies comprise both naturaltissue and one or more high-density objects. A principal category ofsuch objects is represented by intentionally introduced objects formaintaining or improving the quality of human or animal life, such asobjects being in the form of surgical implants made from metal or othersubstances, reconstruction screws, plugs filled into teeth, coilsintroduced into blood vessels, and various others. A secondary categoryis without limitation formed by high-density markers used for allowing aregistration to match various different data sets. The 3D reconstructingmethods recited supra, and possibly others as well, are suffering fromthe visual artifacts that such high density objects may cause in theirneighbourhood, and which artifacts will lessen the quality of theeventual image, and thereby diminish its value for diagnostic, curativeand other purposes. In consequence, it would be advantageous to have anapproach for suppressing such artifacts. The inventors have recognizedthe advantage of suppressing the high-density object(s) from theprocessing in an early stage of the latter.

SUMMARY TO THE INVENTION

In consequence, amongst other things, it is an object of the presentinvention to provide an image generating method that does not sufferfrom the above artifacts from the high-density bodies introduced.

A further object of the present invention is to supplement the imagefrom the natural tissue in a secondary processing stage with the imageof such high density object(s) whilst still avoiding the generation ofthe above artifacts.

By itself, U.S. Pat. No. 4,590,558 discloses a method for removingobjects from CT images, wherein an operator defines a “rub-out” regionthat encompasses the object to be removed, whereafter the rub-out regionis subjected to an averaging function. The operations by the operatorclearly necessitate appreciable effort by a skilled worker who mustcarefully consider the possible location and shape of the high-densityobject, and on the basis thereof set the rub-out region. The prior artapproach will nevertheless be prone to human and other errors, and anautomatic procedure would therefore be much preferred.

Furthermore, U.S. Pat. No. 6,094,467 requires, next to the standardimaging apparatus an additional hardware facility with many narrow-beamdetection facilities for determining the extent, and in particular, theboundaries of high attenuation objects and for thereby reducing theartifacts that the high attenuation objects would cause, withoutremoving the high attenuation objects from the image. The additionalplurality of narrow beam detection facilities represent additional costand additional control operations, and in consequence, the presentinventors have undertaken to derive all necessary information from asingle measuring system.

The invention also relates to a method being arranged for implementingthe apparatus, and to a computer program and to a computer programproduct comprising instructions for controlling hardware for therebybeing arranged for implementing the method. Further advantageous aspectsof the invention are recited.

BRIEF DESCRIPTION OF THE DRAWING

These and further aspects and advantages of the invention will bediscussed more in detail hereinafter with reference to the disclosure ofpreferred embodiments, and in particular with reference to the appendedFigures that show:

FIG. 1, a flow diagram of a procedure according to the presentinvention;

FIGS. 2 a–2 e, the principle of suppression of the high-density objects;

FIG. 3, a 3D reconstruction image of metallic screws in a human kneeenvironment, executed whilst including the suppression and superimposingmethod for high-density objects according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 illustrates a flow diagram of a procedure according to thepresent invention. In block 20, the procedure is started and thenecessary hardware and software facilities are assigned. In block 22 themeasuring apparatus executes a measuring scan. The prime consideredtechnology is 3D-XRAY (3D-RX), but 3D-CT (computer tomography)technology could be enhanced as well. In block 24, the system determineswhether all intended scans have been executed. If no, the next scan ismade after an appropriate rotation of the apparatus; if yes, theprocedure proceeds to block 26. In block 26, a ramp filter in thedirection of rotation is applied to the projection. The setting of theramp may be done once and for all, it may be done on the basis ofstatistical processing, such as based on the assumption that the areacovered by the high density object(s) is generally small, or it may bedone in a heuristic or even intuitive manner, or it could be effected byan operator person. Inasmuch as the high-density objects would generallycause a much greater attenuation that the tissues, the precise settingof the discrimination threshold is not critical.

Next, in block 28, the filtered out high-density object(s) are segmentedto get a 3D reconstruction thereof. Next, in block 30 the high-densityobjects are suppressed in the original projection. Advantageously, theyare then replaced by one or more gray values. This may be done by linearinterpolation between the neighbouring pixels, by replacing eachsuppressed object by a single standard value, or by some otherappropriate steps. In block 32, the ramp-filtered projections are thensegmented a second time, but now without the high-density object(s). Astraightforward approach would be to use exactly the same discriminationthreshold as for the input values to block 28.

Finally, in block 34, the segmented one or more high-density objectsfrom block 28 are superimposed on the segmentation result of theremainder. Finally, in block 36, the procedure is terminated, and theassigned facilities are relinquished again. The flow chart represents invarious respects a simplification. For example, no escape procedure hasbeen shown other than at the successful termination of the processing.Further, a trial and error procedure could be used for effectivelysetting the ramp threshold.

FIGS. 2 a–2 e, illustrate the principle of suppression of the highdensity objects. The method will be described for 3D-RX. A 3D-imagingmodality bases on a number of projections acquired during a rotationalrun with a motorized C-arm system. FIG. 2 a shows one simplifiedprojection image. Therein, items 1 to 3 are high-density objects orso-called bullets used for registration of different data sets. Afterregistration, the bullets must be eliminated for a 3D-reconstruction inorder to avoid artifacts. When measuring an intensity curve across theprojection, at the position of the bullet a signal of high-intensityarises. This signal has to be suppressed and may be filled with grayvalues derived from the neighbourhood that comprises surroundingstructures with lower densities.

The suppression of the high-density object(s) is effected by usingramp-filtered projections, which at the same time improves andsimplifies the reconstruction. In this respect, FIG. 2 a shows aprojection image with bullets 1, 2, 3 clearly visible in the form ofblack circles. In a first processing step, all projections areramp-filtered and used for a 3D-reconstruction through back-projectionwith the well-known Feldkamp algorithm. Also, the bullets, or for thatmatter, other high-density objects, are segmented in the3D-reconstruction, resulting in the picture of FIG. 2 b. Theramp-filtering results in sharper edges of the bullets, and the3D-reconstruction enhances the contrast. Through so acting, thesegmentation of the high-density objects from the surrounding tissue orstructures of their neighbourhood can be effected much better. Next, thebullets can be discriminated by a simple threshold, so that inconsequence only the bullets themselves are visible in the form ofpoints with encircling rings. The latter represent the shadow of thebullet in question.

Now, in a third processing step, the remaining bullets are forwardprojected into ramp-filtered versions of the original projections inorder to mark the search regions for the respective bullet borders,leading to the result illustrated in FIG. 2 c. After the aboveprocessing, the detected borders of the bullets will be forwardprojected into the original projections, resulting in the image shown inFIG. 2 d. In the fourth processing step, the bullets are suppressed bysubstituting them by the gray values of the structures in theirsurrounding neighbourhood, such as by linear interpolation between therespective entry point and the corresponding exit point of the bullet inquestion. Subsequently, a new 3D-reconstruction is performed throughagain applying the Feldkamp algorithm with ramp-filtered projections.Thereby, the bullets are reduced or even eliminated, such also includingremoval of the artifacts caused by the bullets, leading to the imageshown in FIG. 2 e.

FIG. 3 illustrates a 3D reconstruction image of metallic screws in ahuman knee environment, executed whilst including the suppression andsuperimposing method for high-density objects according to the presentinvention. The reconstruction clearly shows the knee joint made up oftwo major bones and the knee-cap, and also two screws used forclinically fixating bone parts to each other. Furthermore, clearly, noartifacts can be seen in the image. The result is attained throughincluding the high-density suppression method in the 3D-reconstruction.The final image is reconstructed by matching the 3D-data set of the bonestructures with the 3D-reconstruction of the segmented screws.Furthermore, the gray values of the screws has been adapted to show bothscrews and bones at the same time. Such adapting may be represented byreducing the overall dynamic representation of the various objects, andin particular, of the high-density objects. Such simultaneouspresentation allows a better diagnosis of the region between the screwsand the surrounding bone structures. Artifacts caused by thehigh-density implants can be much reduced now.

The inventors have found that the results according to the presentinvention are much better when using ramp-filtered projections incombination with a 3D-reconstruction like through the Feldkampalgorithm. The edges of the high density bodies will generally be muchsharper and the contrast is improved. Thereby, a segmentation of theimplants from the surrounding structures can be done much better. Thesegmentation can be easily performed with a simple threshold. This is animportant aspect of the present invention, inasmuch as segmentationalgorithms are often quite complex and thereby, time-consuming.

1. An apparatus for generating images of a subject on the basis of3D-reconstructions from 3D-XRAY or 3D-Computer Tomography measurements,which subject comprises both natural tissue and one or more high-densityobjects, said apparatus comprising a measuring facility for executingsaid measurements, a distinguishing facility for distinguishing said oneor more high-density objects and executing a separating procedurethereon for generating an image of regions of said natural tissue, saidapparatus being characterized by comprising: a ramp-filtering facilityfor applying a ramp filter in the direction of rotation to variousprojection measurements and a back-projecting facility fed by saidramp-filtering facility for back-projecting the various filteredprojections into a 3D-volume reconstruction; a segmenting facility fedby said back-projecting facility for segmenting in said 3D-volumereconstruction said one or more high-density objects by a thresholdingprocedure and a forward projecting facility fed by said segmentingfacility for executing a forward projection of a shadow(s) of thesegmented one or more high-density objects onto the ramp-filteredprojection, whilst marking borders of said one or more high-densityobjects in the ramp-filtered projections; a suppressing facility fed bysaid forward projecting facility for suppressing the reconstructed oneor more high-density objects from the original projection measurementsand said suppressing facility is operative for executing an appropriatesubstitution of gray values derived from a physical neighbourhood ofsaid one or more high-density objects instead of said one or morehigh-density objects in question; and a retro-coupling facility fed bysaid suppressing facility for executing a back-projection of the variousfiltered projections with corrected profiles through exclusion of thesuppressed one or more high-density objects and outputting areconstruction result.
 2. An apparatus as claimed in claim 1, andfurthermore comprising a superimposing facility fed by said forwardprojecting facility for receiving said one or more high-density objectsfor superimposing thereof onto said reconstruction result.
 3. Anapparatus as claimed in claim 1, and furthermore comprising adaptingmeans for relatively adapting the gray values of said one or morehigh-density objects and said natural tissue in a predetermined grayvalue range to show both of said one or more highdensity objects andsaid natural tissue at same time.
 4. A method of generating images of ahuman or animal body on the basis of 3D-reconstructions from 3D-XRAY or3D-Computer Tomography measurements, which body comprises both naturaltissue and one or more high-density objects, said method comprising thesteps of executing said measurements, distinguishing said one or morehigh-density objects and executing a separating procedure thereon forgenerating an improved image of regions of said natural tissue, saidmethod being characterized by comprising the steps of: applying a rampfilter in the direction of rotation to various projection measurementsand back-projecting the various filtered projections into a 3D-volumereconstruction; in said 3D-volume reconstruction, segmenting said one ormore high-density objects by a thresholding procedure and executing aforward projection of a shadow(s) of the segmented one or morehigh-density objects onto the ramp-filtered projection, whilst markingborders of said one or more high density objects in the ramp-filteredprojections; suppressing the reconstructed one or more high-densityobjects from the original projection measurements whilst executing anappropriate substitution of gray values derived from a physicalneighbourhood of said one or more high-density objects instead of saidone or more high-density objects in question; and secondarily executinga back-projection of the various filtered projections with correctedprofiles and thereby without the suppressed one or more high-densityobjects.
 5. A computer readable medium containing instructions forcontrolling a computer system to perform the steps of a method ofgenerating images of a subject on the basis of 3D-reconstructions from3D-XRAY or 3D-Computer Tomography measurements, which subject comprisesboth natural tissue and one or more high-density objects, said methodcomprising the steps of executing said measurements, distinguishing saidone or more high-density objects and executing a separating procedurethereon for generating an improved image of regions of said naturaltissue, said method furher comprising the steps of: applying a rampfilter in the direction of rotation to various projection measurementsand back-projecting the various filtered projections into a 3D-volumereconstruction, in said 3D-volume reconstruction, segmenting said one ormore high-density objects by a thresholding procedure and executing aforward projection of a shadow(s) of the segmented one or morehigh-density objects onto the ramp-filtered projection, whilst markingborders of said one or more high density objects in the ramp-filteredprojections; suppressing the reconstructed one or more high-densityobjects from the original projection measurements whilst executing anappropriate substitution of gray values derived from a physicalneighbourhood of said one or more high-density objects instead of saidone or more high-density objects in question; and secondarily executinga back-projection of the various filtered projections with correctedprofiles and thereby without the suppressed one or more high densityobjects.